1. Field of the Invention
The present invention relates to novel calcium carbonate fillers, and, more especially, to calcium carbonate filled polymeric compositions, notably the rigid polyvinyl chlorides.
2. Description of the Prior Art
It has long been known to this art to use calcium carbonate particles as filler materials for elastomer or other polymer compositions. Such calcium carbonate is obtained from natural deposits, thereafter being finely ground (in either moist or dry state, but, if moist, followed by drying), or is obtained via chemical precipitation and drying of the precipitate. It too is known that the dispersibility of fillers in a polymeric matrix widely varies, depending upon the particle size of the filler, and characteristically decreaases with diminishing particle sizes; thus, to utilize very finely divided particulates in polymeric matrices, it is necessary to extend the mixing times therefore or the shearing effectiveness of the mixer. This is of course counterproductive as regards the heat-sensitive polymers, because any improvement in mechanical properties by reason of the filler content is lost due to concomitant thermal degradation of the polymer.
And different solutions have already been proposed to the art to offset the lack of affinity of calcium carbonate to materials which are essentially oleophilic, such as the typical natural and synthetic polymers. Thus, it is known to modify the surfaces of filler particles by treating them with a solution of polymer in solvent and then drying the treated particles; other known processes involve the preliminary treatment of the particulate filler with monomers, followed by polymerization thereof, and a third type of process features the fixation on the surfaces of the filler particles of oleophilic compounds, such as fatty acids. Finally, yet another category of process features the treatment of a calcareous filler to improve its wettability by means of hydrocarbons and plastic materials by contacting the same with alkylarylsulfonic acids or salts, the calcium salt thereof being essentially insoluble in water, in an amount at least equal to that required to form monomolecular layer or coating covering the entire surface of the particles or grains, as described in published French Application No. 2,138,300. This publication specifically relates to calcium carbonate powders having a low specific surface, on the order of 0.5 to 3 m.sup.2 /g (BET method).
However, the aforesaid proposed solution does not suffice for all possible applications, either because of the filler itself or the particular utilization thereof.
It is also known that the incorporation of mineral fillers in a polymer matrix improves certain mechanical properties, while other properties may be diminished. This is particularly true in the case of polyvinyl chloride (PVC), the notched impact strength thereof being improved by the incorporation of a mineral filler such as calcium carbonate, but such improvement being obtained at the expense of the unnotched impact strength, which decreases. Furthermore, the aforenoted opposite effects become increasingly pronounced the smaller the size of the particles. Great need therefore exists for the identification of a calcium carbonate filler capable of imparting a high notch impact strength to a reinforced polymeric composition, without concomitantly giving rise to brittleness in the absence of a notch.